142 The American Naturalist. [February, 
that (as easily can be verified) the chele have, as in the crab, a 
groove on the second segment, while the other legs present a free 
joint at the corresponding point, making thus two segments in 
place of one, or seven in all in place of six in the chelz. 
Considering the relations of the crab to the Macrurans, there 
seems no doubt that the second segment in the leg of the former 
represents the fused second and third segments in the latter; the 
“plane of rupture” corresponds in ‘position with the free joint 
between the second and third segments of the leg in the lobster 
or crayfish. 
The appearances seen on sectioning this plane, may, I judge, 
be explained as a modification of a former free joint; the double 
membrane and line of discontinuity of exoskeleton representing 
the invagination of body-wall seen at an ordinary movable joint 
where tendons for attachment of muscles are formed. 
As the lobster appears to have the power, though feebly de- 
veloped, of throwing off the legs at the free joint between the 
second and third segments, and as this power is better developed 
in the chelz, where fusion of the above segments has taken place, 
may we not suppose that the more perfect and ready autotomy 
in the crab has been gradually derived from the former conditions 
as a “ change of functions” took place from a movable joint to a 
definite “ plane of rupture” ? 
That this “plane of rupture” is found in the Megalops (as I 
infer from examinations of alcoholic specimens) does not, I think, 
invalidate the above conclusion. 
Of the two grooves seen on the second segment of the crab's leg it is the proximal 
one that corresponds to the rupture plane of the lobster’s chelze; the distal one being 
represented in the lobster by a deep depression, possibly bearing some relation to the 
exopodite. 


